1. Field of the Invention
The invention generally relates to platelet/flake magnesium oxide, uses, and methods of making the same. One such use relates to water-insoluble ceramic materials made from the platelet/flake magnesium oxide.
2. Brief Description of Related Technology
There is no direct affinity between magnesium oxide (MgO) and magnesium salts. However, in the presence of water a chemical bond forms between magnesium oxide and magnesium salts (e.g., magnesium chloride or magnesium sulfate) to form a slurry which cures to a ceramic material (e.g., magnesium oxychloride or magnesium oxysulfate). Magnesium oxychloride and magnesium oxysulfate ceramic materials are particularly desirable for use as cements because they exhibit excellent strength characteristics and excellent fire-retardant properties. Furthermore, such materials, when reinforced with glass fibers may be used to produce fire-retardant molded articles, such as automotive parts and building panels.
One method of making magnesium oxychloride and oxysulfate cements is described in U.S. Pat. No. 4,158,570, the disclosure of which is hereby incorporated herein by reference. The ""570 patent teaches that magnesium oxychloride and oxysulfate cements are prepared by mixing magnesium oxide (also referred to as magnesia) with concentrated aqueous solutions of magnesium chloride and magnesium sulfate, respectively, typically while employing high shear mixing conditions.
According to the ""570 patent (column 2, lines 6-10), a hydrated magnesium chloride (MgCl2.6H2O) solution typically contains 60 percent by weight (wt. %) to 85 wt. % solids, whereas, a hydrated magnesium sulfate (MgSO4.7H2O) solution typically contains 50 wt. % to 75 wt. % solids. However, magnesium chloride solutions having a specific gravity of above 28xc2x0 Baumxc3xa9 (corresponding to a MgCl2.6H2O solids concentration of about 59 wt. %), and magnesium sulfate solutions having a specific gravity of above 34xc2x0 Baumxc3xa9 (corresponding to a MgSO4.7H2O solids concentration of about 48 wt. %) are discouraged because the formed slurry would have an undesirably high viscosity and would be difficult to cast and/or mold. See ASTM A88.2-1952, for example. The magnesium oxychloride and oxysulfate materials made in accordance with these teachings, however, exhibit physical characteristics, described below, which could be improved upon.
The specific gravity of a solution may be calculated on the Baumxc3xa9 (Be) scale, expressed in degrees, by the following formula:
Be=mxe2x88x92(m/s)
wherein m is 145 for liquids heavier than water, and m is 140 for liquids lighter than water, and s is the specific gravity. Hence, 28xc2x0 Baume corresponds to a specific gravity of about 1.25, and 34xc2x0 Baumxc3xa9 corresponds to a specific gravity of about 1.4.
Generally, water should not be combined with a magnesium 25 oxide prior to adding a magnesium salt because the oxide will undesirably hydrate to magnesium hydroxide (Mg(OH)2). Magnesium hydroxide is insoluble in water and does not combine readily with magnesium salts to form the desired cementitous material. Thus, the magnesium oxide should be admixed with an aqueous magnesium salt solution. Accordingly, the ""570 patent teaches that the aqueous magnesium chloride solution used to make a magnesium oxychloride cement is prepared by mixing hydrous magnesium chloride with water, phosphoric acid (H3PO4), and sodium hexametaphosphate ((NaPO3)6). The magnesium oxide is then added to the formed solution in a molar ratio of about 3:1 to about 8:1 to result in a slurry or paste-like material which, when cured (i.e., set and molded to a desired shape), produces a magnesium oxychloride ((MgO.MgCl2.6H2O) material.
Similarly, the aqueous magnesium sulfate solution used in the manufacture of the magnesium oxysulfate cement is prepared by mixing magnesium sulfate with water, phosphoric acid (H3PO4) and sodium hexametaphosphate ((NaPO3)6). The magnesium oxide is then added to the solution in a molar ratio of about 3:1 to about 14:1 to result in a slurry or paste-like material which, when cured, produces a magnesium oxysulfate ((5MgO.MgSO4.H2O) material. Curing, in either case, can occur at room temperature or at elevated temperatures.
The ""570 patent further teaches the importance of using high shear blending to bring about and ensure a de-agglomeration and de-flocculation of the magnesium oxide particles and to more evenly disperse the magnesium oxide throughout the magnesium salt solutions (chloride or sulfate). The ""570 patent warns that if high shear mixing is not employed during the blending step, the resultant cured materials will not possess the necessary water insoluble characteristic and will have other undesirable physical and structural characteristics.
High shear mixing, however, imparts heat (i.e., temperatures of about 130xc2x0 F. (about 54xc2x0 C.)) to the oxide/salt mixture which can cause premature cure of the cementitous mixture. In order to avoid such premature cure, the ""570 patent teaches to admix the magnesium oxide with the magnesium salt solution in a controlled, stage-wise manner, wherein a first portion of the magnesium oxide initially is added to the solution under low shear mixing conditions and thereafter, a second portion and additional portions (e.g., the remainder) are successively admixed with increasingly high shear mixing until all of the oxide has been added.
Generally, the magnesium oxide used to prepare the cement is conventionally manufactured by thermal decomposition or chemical reaction of various magnesium compounds such as, for example, magnesite ore (e.g., magnesium carbonate (MgCO3)), magnesium hydroxide, and magnesium chloride. One such method, practiced by Martin Marietta Magnesia Specialists, Inc., at its Woodville, Ohio plant, generally includes calcining dolomitic limestone (CaMg(CO3)2)) at high temperature to produce calcined dolomite ((CaO.MgO) or dolime, which reacts with a magnesium chloride-rich brine (CaCl2.MgCl2.H2O) solution to produce a slurry having solid particles of insoluble magnesium hydroxide (Mg(OH)2) and a liquid phase of calcium chloride (CaCl2). The solid magnesium hydroxide then is separated from the liquid calcium chloride carrier and further calcined to form a various grades of magnesium oxide, such as reactive, light-burned magnesium oxide to unreactive, dead-burned magnesium oxide.
Other prevalent methods of manufacturing such conventional magnesium oxides, including seawater methods, generally are described in Jackson, L. C. et al., xe2x80x9cMagnesium Compoundsxe2x80x9d in: Encyclopedia of Chemical Technology. Vol. 15, pp. 675-722 (1995), the disclosure of which is hereby incorporated herein by reference.
At a microscopic level, the formed conventional magnesium oxide is a solid having a spherical shape, generally. When this magnesium oxide is mixed with a high specific gravity solution, such as a magnesium salt solution, the solution fills interstitial voids or spaces present between closely packed magnesium oxide spheres. The heat imparted to the solution during the mixing step(s) causes the solution to expand and, thereby, fracture the spherical particles before the desired cement-forming reaction between the magnesium oxide and salt is complete. Hence, the magnesium oxide never fully dissolves into the solution. The undissolved magnesium oxide has a natural tendency to hydrate to an insoluble magnesium hydroxide when the salt/oxide material is eventually exposed to water. Additionally, conventional magnesium oxychloride cements hydrolyze to produce free magnesium chloride (MgCl2) which is highly corrosive. The presence of magnesium hydroxide and free magnesium chloride results in a structurally inferior material having fair to poor physical characteristics when compared to non-ceramic based cements.
To improve the physical characteristics and structural integrity of ceramic-based cements, the prior art has suggested the addition of various additives, such as water-soluble phosphates. For example, the ""570 patent teaches that the presence of polyphosphates, such as sodium hexametaphosphate, prevents the magnesium salt from undesirably precipitating when the solution is cooled to temperatures less than ambient. Additionally, use of phosphate additives improves the wet strength of the formed cement, and reduces material contraction often experienced during the curing step. Despite the use of phosphate additives, however, the formed cement exhibits low dry strength and low wet strength characteristics, and undesirably hydrates to magnesium hydroxide when exposed to water.
In view of the foregoing, it would be desirable to provide a magnesium oxychloride or magnesium oxysulfate ceramic material which when, exposed to water or other environmental conditions (e.g., high concentrations of ozone), does not result in the undesirable formation of water-insoluble magnesium hydroxide and/or the corrosive magnesium chloride. Furthermore, it would be desirable to provide a magnesium oxide-based ceramic material free of one or more prior art additives (e.g., phosphates). Additionally, it would be desirable to provide a magnesium oxychloride or magnesium oxysulfate ceramic material having superior physical characteristics and superior structural integrity compared to conventional magnesium oxychloride or oxysulfate ceramic materials.
It is an objective of the invention to overcome one or more of the problems described above.
Accordingly, the invention provides a ceramic material, and a method of making the same, comprising a reaction product of (a) a magnesium oxide having a platelet or flake structure (hereinafter xe2x80x9cplatelet/flakexe2x80x9d), a bulk density of about 30 pounds per cubic foot (lbs/ft3) (about 0.48 grams per cubic centimeter (g/cm3)) to about 70 lbs/ft3 (about 1.12 g/cm3), and a particle density of about 215 lbs/ft3 (about 3.45 g/cm3) or less, and (b) an aqueous solution comprising a magnesium salt selected from the group consisting of magnesium chloride, magnesium sulfate, and mixtures thereof, wherein the magnesium salt solution has a specific gravity of about 1.18 to about 1.4.
The platelet/flake magnesium oxide used to make the ceramic material can be prepared from a magnesium oxide ore or from a brucite ore, for example. The platelet/flake magnesium oxide can be prepared from magnesium oxide ore particles, preferably having a particle size of about xc2xc inch to about five inches, by heating the particles at a temperature and for a time period sufficient to remove contaminants therefrom. Thereafter, the particles are re-hydrated with water and then heated to vaporize the water resulting in expanded ore particles having a hollow shaped structure. The hollow shaped structures then are ground to produce the platelet/flake magnesium oxide.
Alternatively, the platelet/flake magnesium oxide can be prepared from brucite ore particles, preferably having a particle size of about xc2xc inch to about five inches, by heating the particles at a temperature and for a time period sufficient to convert the brucite to magnesium oxide and to remove contaminants therefrom. Thereafter, the converted, magnesium oxide particles are heated to vaporize water present in the particles resulting in expanded magnesium oxide particles having a hollow shaped structure. The hollow shaped structures then are ground to produce the platelet/flake magnesium oxide.
Regardless of the particular starting material, a platelet/flake magnesium oxide can be prepared by a method that includes the steps of heating high-purity magnesium oxide particles to vaporize water present in the particles, resulting in expanded particles having a hollow shaped structure, and shaping the hollow shaped structure to produce the platelet/flake magnesium oxide.
Further objects and advantages of the invention may become apparent to those skilled in the art from a review of the following detailed description, taken in conjunction with the examples, the drawing figures, and the appended claims.